A common design of light sources for projectors uses a phosphor wheel to generate light of different colours sequentially. Typically, a blue laser excites the phosphor wheel to generate green or red colour light. The phosphor wheel normally has some fan segments which contain different types of phosphor to convert the exciting blue light to a green, yellow or red colour.
The phosphor wheel can also have one or more gaps to pass the blue source light unconverted. A reflective type of phosphor wheel is often used, such that the excitation light and the emitted light stay on one side of the phosphor wheel. Then, a dichroic is used to separate the excitation light and emitted light.
An example of a known such design is shown in FIG. 1A, comprising a dichroic element 100 and a phosphor wheel 200. The phosphor wheel comprises a disc portion 201 and a motor 202 causing the disc portion 201 to rotate. The source blue light 301 is passed by the dichroic element 100 and the passed blue light 302 (also referred to as the excitation light) is focused on the disc 201 of the phosphor wheel 200. The disc portion 201 has at least one segment comprising a phosphor. The blue excitation light 302 incident on such segments is absorbed by the phosphor and converted light 303 (typically green or red) is emitted. To extract the green or red light 303, the dichroic element 100 is used. The dichroic element 100 reflects the emitted light 303 and the reflected light 304 is output. The disc portion can have multiple segments with different phosphors, such that the output light 304 may be provided with different colours in sequence, as the disc portion 201 is rotated.
The projector normally requires red, green and blue light to generate images. With blue excitation light 302, the phosphor wheel would normally generate green and red output light 304. However, the design of this light engine demands that the dichroic element 100 passes blue light to reach the phosphor wheel 200. Thus, the dichroic element 100 cannot normally reflect any blue light received.
Some known technologies create blue light in a different way. The disc portion 201 of the phosphor wheel 200 is designed with a gap segment. The gap segment allows the source light 304 to pass through the phosphor wheel (not being reflected) and provides secondary output light 305 of the same colour as the source light 301, for example blue light.
An alternative, known design of light engine is also shown in FIG. 1B. Similar light engine designs are discussed in U.S. Pat. No. 7,070,300. This light engine achieves the same function as that shown in FIG. 1A and mostly uses the same components. As a result, the same reference numerals have been used to identify the same features as shown in FIG. 1A. However, the arrangement of the components is different in FIG. 1B in comparison with FIG. 1A. Moreover, the dichroic element 101 is different from the dichroic element 100 of FIG. 1A.
The source light 301 is reflected by the dichroic element 101, causing excitation light 302 to be incident on the disc 202 of the phosphor wheel 200. The emitted light 304, being of a different colour from the excitation light 302, is then passed by the dichroic element 101. Again, the provision of light having the same colour as the source light 301 is achieved by the use of a gap segment in the disc portion 201 of the phosphor wheel 200. This allows secondary output light 305 of the same colour as the source light 301 to be provided.
Thus, for the designs shown in FIGS. 1A and 1B, there are two output light paths. For example, one is provided for green and red light and a second for blue light. This results in additional components, increasing size, complexity and cost.